Several procedures are known for rendering images containing elements defined as three dimensional data. A known approach to generating images of photo-realistic quality is to consider reflections between all elements simultaneously. The light emission of any given element is considered as being dependent upon the sum of contributions from all other elements, and a set of equations is defined that represents these interactions. The light emission values for all the elements, are then determined simultaneously by solving a system of equations.
This procedure is known as radiosity simulation. The system of equations is usually extremely large, and several refinements to radiosity simulation have been established in order to make implementation of this method practical for scenes containing large numbers of elements.
A known advantage of radiosity is that once the system of equations has been solved, and light emission values determined, the light emission of elements may be considered as view-independent, resulting in a separate radiosity rendering process which is capable of rendering a view from any position. The high efficiency of radiosity rendering makes radiosity particularly suitable for demanding applications, such as generating long sequences of image data frames for film or video, or generating image data in real-time.
Lighting sources may be superimposed upon a radiosity rendered scene, and graphics systems are known which support direct light source simulations, such as those using Phong shading, so that lighting effects may be adjusted in real-time. High quality lighting algorithms, such as ray tracing, are difficult to calculate in real-time, and so are unsuitable for providing changing lighting conditions at a speed that matches that of radiosity rendering.
Complex real time lighting effects may be achieved within radiosity by splitting fixed lighting combinations into light banks, whose intensities may then be varied when rendering. Thus, although the light sources are fixed in position, light banks may be located at different positions, and their variation in intensity, as well as generating changes in colour shading, can result in dramatic changes to the shadows in the image. The addition of light flux components incident upon a surface is linear. Thus it is possible to pre-compute a radiosity simulation for a scene which is illuminated by a plurality of light banks. When three light banks are used, whose intensity is to be varied, three radiosity solutions may be calculated, one for each light bank. The results of the solutions may then be linearly combined in response to fader settings from a lighting mixer at the time of rendering.
This approach is described in "Interactive Design Of Complex Time-Dependent Lighting" by Julie Dorsey, James Arvo and Donald Greenberg, in IEEE Computer Graphics And Applications volume 15, number 2, March 1995. Although this approach provides a high degree of efficiency at the time of rendering, the radiosity simulation needs to be performed for each of the light banks that are being used. Radiosity simulation is a time consuming process, and this is multiplied by the number of light banks whose contributions are to be varied.